Organic shale reservoirs have very low permeabilities, often necessitating significant fracturing operations to promote efficient production. The stimulated fracture system produced by such operations will be strongly influenced by the extensive horizontal laminations that pervade shale reservoirs. More specifically, these laminations will strongly influence the hydraulic fracture height because of the differences in rock mechanical properties normal and parallel to the bedding planes. In order to accurately predict fracturing height from logs in this environment, these mechanical property differences need to be accurately characterized and taken into account.
During oil and gas exploration and production, many types of information are collected and analyzed. This information is used to determine the quantity and quality of hydrocarbons in a reservoir, and to develop or modify strategies for hydrocarbon production. One type of information that is routinely collected is acoustic log data. In acoustic logging an acoustic source (transmitter) is positioned at a distance from an array of spaced-apart receivers. An acoustic signal is transmitted by the acoustic source to propagate along the borehole wall and be received at the receivers. Measurements are repeated every few inches as the tool descends or ascends in the borehole. The received acoustic signals may be processed to determine first-arrival times or even frequency-dependent (and propagation mode-dependent) wave slowness values to be stored and/or communicated to the surface.
The slowness values for certain propagation modes (compressional wave propagation, shear wave propagation, and Stoneley wave propagation) can be converted into estimates of corresponding stiffness coefficients, but these stiffness coefficients alone result only in an incomplete characterization of the formation's mechanical properties. The well-known ANNIE model postulates that certain empirically-observed relationships may be used to estimate the remaining stiffness coefficients and thereby provide a complete characterization of the formation's mechanical properties. It has now been found that, at least for certain formations, ANNIE-based estimates are incorrect.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.